The cartoon in question certainly pushed the boundaries of good taste and fair comment. It was crude and explicit, breathtakingly so. And therefore hard to defend.

It depicted South Africa's presumptive next president, Jacob Zuma, unbuckling his trousers as he stands over the female figure of justice being held down by other leaders of the ruling party. "Go for it, boss," one of them is saying to him.

It appeared in this week's South African Sunday Times and was drawn by Zapiro, the country's most prolific, outspoken, best-known and much-awarded cartoonist. It arose out of calls by the ruling ANC for a "political solution" to bring an end to Zuma's looming corruption trial and attacks on the judiciary and justice system by his supporters, who feel he is being persecuted by his political enemies.

And it is an unfortunate reminder that Zuma was last year acquitted of rape.

The reaction has been fierce. The ANC and its allies called it "disgusting" and an "abuse of press freedom". They described it as "a direct assault on the ANC… and its leadership". Other ANC leaders called it racist.

Zapiro, who has impeccable credentials as an anti-apartheid cartoonist, refused to apologise and called on Zuma to say sorry for what he and his supporters were doing to the legal system.

The timing was unfortunate. The cartoon gave the ANC the whip it had been looking for to lash the Sunday Times, its outspoken editor, Mondli Makhanya, and the media in general. It was Makhanya's paper which recently accused President Thabo Mbeki of secretly accepting R30m (£2.1m) from an arms dealer, giving R2m (£140,000) of it to Zuma and the rest to the ANC. Mbeki has strongly denied this.

It was the same paper that caused an outcry last year with a screaming headline calling the minister of health a thief and a liar (also denied). The paper has not been a friend either of Mbeki or of Zuma. And it was the same newspaper that has had a recent run of dubious front-page stories which were quickly knocked down, leaving Makhanya on shaky ground.

The ruling party's criticism was to be expected. Zapiro has infuriated them by drawing Zuma with a permanent shower on his head, reminding us constantly of his remark in the witness box that he took a shower after having sex to avoid HIV/AIDS infection. ANC leaders have repeatedly said in recent months that the media is abusing its freedom and have called for a statutory media tribunal to tighten things up.

More serious critics, however, have point to other issues. Is it fair to invoke a trial in which Zuma was acquitted? Is this not playing to a stereotype of black male sexuality? Is it insensitive in a society with a serious problem of violence against women? One writer, Xolela Mangcu, went so far as to say: "There is a race war waiting to happen in this country, and people such as Zapiro will have played no small part in fanning the flames."

It is always cartoons like this – the ones hard to justify – that need defending. It is not easy in a new, young and fragile democracy to defend the right of a cartoonist to be tasteless, demeaning, even unfair. But it has to be done.